In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, the print quality will be inconsistent. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is dried on the blanket and then transferred to media. Such a printer avoids the changes in image quality, drop spread, and media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is jetted on to an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality the ink drops jetted onto the blanket must spread and not coalesce prior to drying. Otherwise, the ink images appear grainy and have deletions. The lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface. In order to facilitate transfer of the ink image from the blanket to the media substrate, however, a blanket having a surface with a relatively low surface energy is preferred. These diametrically opposed and competing properties for a blanket surface make selections of materials for blankets difficult. Reducing ink drop surface tension helps, but the spread is still generally inadequate for appropriate image quality. Offline oxygen plasma treatments of blanket materials that increase the surface energy of the blanket have been tried and shown to be effective. The benefit of such offline treatment may be short lived due to surface contamination, wear, and aging over time.
Applying a coating material to the blanket can facilitate the wetting of the blanket surface with ink drops and the release of the ink image from the blanket surface. Coating materials have a variety of purposes that include wetting the blanket surface, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and/or aiding in the release of the printed image from the blanket surface. Reliably forming a coating layer on a blanket surface is a challenge. If the coating is too thin, it may fail to form a layer adequate to support an ink image. If the coating is too thick, a disproportionate amount of the coating may be transferred to media with the final image. Image defects arising from either phenomenon may significantly degrade final image quality. Consequently, development of blanket surfaces that provide high energy surfaces for image formation and then reduce the surface energy for image transfer without adding the issues of coating the blanket is desirable.